The objective of this proposal is to improve our understanding of electronic and dynamic aspects of the mechanism of cooperativity in hemoglobin and to increase our knowledge of the nature and implications of motions in systems of biological interest. First, molecular orbital calculations will be used to elucidate the electronic structure and properties of the heme group interacting with a variety of ligands and how these properties are altered by the protein. Using the same methodology, we will investigate the effect of environment on the spectra of the proposed structures for the chromophores in the two photoreversible forms of phytochrome. Second, as part of our continuing efforts to understand the relationship between the structure (both) electronic and steric) of hemoglobin and the kinetics of ligand binding, we will develop theories which incorporate a realistic description of the entire reaction path of a ligand as it moves from solution to the binding site. Finally, we will formulate models of increasing sophistication for the purpose of extracting detailed motional information from nuclear magnetic resonance relaxation and fluorescence depolarization experiments on proteins and membranes. Special emphasis will be placed on evaluating the uniqueness of such information.